Process of producing wood pulp of high alpha cellulose content



Patented June 9, 1931 No Drawing.

UNITED STA GEORGE A. mcn'rnn AND MILTON o.

TO BROWN comm, or BERLIN,

SCH UR. OE BERLIN, HAMPSHIRE, ASSIGN'ORS NEW HAMPSHIRE, A CORPORATION OF MAINE PROCESS PRODUCING WOOD PULP OF HIGH ALPHA. CELLULOSE CONTENT This invention relates to the production of wood pulp of high alpha cellulose content, and has for its object the production of a pulp possessing optimum characteristics both for manufacture into paper and for use as a raw material for the preparation of cellulose derivatives.

' bleach content Heretofore, pulp of high alpha cellulose content has been produced by the general process disclosed and claimed in our application for patent, Serial No. 716,154, filed May 27, 1924. In accordance with that process, a raw pulp, e. g., pulp, is subjected to a controlled oxidizing or chlorinating treatment which does not whiten the pulp appreciably, but, as amatter of fact, generally causes a deepening of the color of the pulp from a pale cream or light brown to a Iyellow or light orange, this color generation mation of colored addition or oxidation products from the ligneous content of the pulp. Thetreated pulp is then digested in an alkaline'liquor until a considerable portion of its non-alpha cellulose component, including the chlorinated or oxidized ligneous reaction products of the previous treatment, has been removed by dissolution, whereupon the pulp is washed and treated in a liquor containing bleach in sufficient amount to whiten it substantially to ultimate whiteness.

Inasmuch as the oxidizing or chlorinating treatment as described in said application effects what may be termed but a mild or selective reaction upon the ligneous content of the pulp, it is necessary to use 2% or more of bleach powder, based on the bone-dry weight of fiber, in the final bleaching treatment to remove sufiicient ligneous ,and coloring content from the alkaline-digested gill to whiten it to the desired point. uc bleach liquor, although considerably less in than that necessary to whiten sulphite pulp, yieldsa product I ing a copper number falling 7 between 1.7 and 2.5, and a' solubility in'boillng 3% caustic .solution of 5% to 7 %,--indicating a low oxycellulose content as compared with the usual bleached sulphite pulp. It is desirable, however, further to reduce the copper number and unbleached sulphite;

eing the result of the forance with our present I bleach in the first step,

usually hav- Application filed April 19, 1927. Serial No. 185,019.

soda solubility so as to enhance the suitability of the product as a raw material not only for the preparation of papers of high stability, comparable with high grade rag papers, but also for the preparation of cellulose derivatives such as nitrocellulose, cellulose acetate, cellulose xanthate, etc-., as the oxycelluloses are nitrated', acetated, or xanthat ed and hence 1ts suita ility as a raw material in creased, by reacting, as will be more fully along with the alpha cellulose to form unhereinafter described, more completely upon the ligneous and coloring content of the raw in a treatment prior to alkaline digesso that after alkaline digestion this content .is largely removed, permitting substantially ultimate whiteness to be finally obtained in a liquor of very low bleach concentration, thus'to produce a final product of low oXycellulose content. That is to say,inlieu of removing much of theligneous content of thepulp in the final bleaching step, we efi'e'ct, in accordprocess, the removal by the combined p p tion,

of most of such content effects of the initial treatment and the alka- I line digestion, so that the final bleaching may be performed in liqu'or of relatively low bleach-content. The initial'treatment is carefully controlled, however, so that although the ligneous and other coloring content is thoroughly reacted upon, the pulp is not bleached or whitened prior to the alkaline digestion, since if this were attem te'd,'it

would be necessary to use considerab y more this not only resulting in the production of oxycelluloses, which because of solution by su sequent alkaline digestion would be reflected in a lower yield of product,

istics of the fiber.

but alsoin a permanent loss of the desired physical and chemical character- It is the object of the present invention to: I

remove most of the ingcontent from the raw pulp "by the com ligneous and other color'- i" i I "usually less than 2% and frequently about 1% of 35% bleach powder based on fiber, in the final bleaching step, to bring the pulp to a high degree of whiteness. Our theory is that, even though the pulp is still highly colored after the first treating step, were it not for coloring matter adsorbed from the dark brown liquor generated during the alkaline digestion, the pulp would needno final bleaching, the coloring matter having been removed by the combined efiects of the initial treatment with chlorinating and/or oxidizing agents, and by the alkaline digestion.

In our application, Serial No. 716,154, we stated that:

We have noted that when unbleached chemical wood pulp is subjected to the action of an oxidizing agent, with a given proportion of the reagent during a given period of time, the first consumption of the oxidizing agent is accompanied by a rise in the proportion of alpha cellulose in the pulp, but that a critical point is reached, so that the further consumption of the oxidizing agent is accompanied by a marked decrease in the proportion of the alpha or resistant cellulose content of the pulp. We have also noted that the copper number decreases with the first consumption of the oxidizing agent up to the critical point, and thereafter rises. Consequently, in carrying on the first step of our process, we employ a relatively small amount of oxidizing agent and terminate the operation when or before this critical point is reached, this taking about thirty minutes under the conditions stated in the specific example. i

In the example cited, we subjected the alkaline-digested pulp to the action of 2% to 4% bleach, based on the pulp. In carrying out the process of the present invention, we pass slightly the critical point defined in that application, so that final bleaching may be effected with less than 2% bleach, based on pulp. This process requires much more operative skill than our earlier one, for ifthe critical point is passed to any substantial extent, the physical and chemical characteristics of the pulp sufi'er.- On the other hand, by using the proper quantity of chlorinating and/or oxidizing agents to pass slightly the critical point so that a high degree of whiteness may be finally. attained with 1% or 2% bleach, "we effect a sufiicient increase of stability, lowering of copper number, and reduction in soda-solubility in the product, to

warrant the increased care necessary in the initial treatment.

' -Our new process for the manufacture of high alpha cellulose fiber represents an advance over the earlier one, for in addition to the decided and important improvement effected upon the product in its stability and in its freedom from oxycelluloses, its viscosity may be better controlled, so that a product of high viscosity and high whiteness, and which is of advantage in the preparation of cellulose esters, may be more readily prepared than by our earlier process.

The new process may be advantageously applied to hard, strong pulps prepared with a sulphite liquorhigh in combined sulphur dioxide, and which normally require the application of large. percentages of bleach. By reacting on the ligneous and coloring matter of such pulps prior to the alkaline digestion, up to a point where but 1% to 2% bleach is required in the final bleaching step, a prodnot having optimum characteristics is invariably obtained. Thus, the product is high in strength and tear, in alpha cellulose content, and in whiteness, yet is very low in copper number and in soda-solubility. The product is even more permanent in whiteness than is the usual alpha cellulose pulp, which latter is much more stable in this respect than the usual bleached wood pulp. For instance, alpha pulp produced by the process of the present invention yellows but little when heated at, say, 212 F. for 24 hours, or when exposed to ultra-violet light,conditions under which sulphite pulp would turn a muddy yellow.

We have found that chlorine water is very selective in its action upon the ligneous content of the pulp, and that if such a reagent is used in suflicient strength for treating raw pulp, substantially all the ligneous content of the pulp may be reacted upon without causing material degradation of its alpha cellulose content. For instance, if raw pulp is treatedwith chlorine water of sufiicient chlorine content, substantially all its ligneous content is reacted upon without material effect upon its final alpha cellulose content. Such treatment does. not bleach or whiten the pulp, but as a matter of fact, tends to deepen its color from a pale cream or light brown to a yellow or light orange. We may, however, use an oxidizing reagent such as hypochlorite, of sulficient strength to react upon substantially all the ligneous content of the pulp, but when we so do, we find it necessary to maintain it distinctly alkaline, as under such conditions a more selective reaction is efiected upon the ligneous matter, which is more selective to oxidation than the alpha cellulose. Or, we may react upon a portion of the ligneous con-" tent of the pulp by chlorine water, and upon the remaining portion by hypochlorite, or vice versa.

We have found that the reaction between raw pulp and chlorine is very rapid up to the critical point, then is very' slow, so that ed for optlmum results will be'consumed and the excess removed during the washing prior to the alkaline digestion. Byusing a moderate excess of chlorine asabove described,

the initial step in our process becomes more easily controllable. This, of course, is an exceedmgly important advantage, especially in a contmuous process, for by adjusting the chlorine input so as to maintain the desired excess in the pretreated fiber, variations in the quality of the raw stock may be closely followed with less danger of overor undertreatment.

A specific example of procedure which may be followed in producing a product of the desired characteristics may be substantially as follows. Wood chips, e. g., spruce, may be digested in the usual acid calcium bisulphite liquor, under the usual conditions, for the necessary period of time to efiect a fiber liberation. It may be preferable, however, to efl'ect fiber liberation in liquors of other combined and free SO contents, for-instance in an acid alkali-metal bisulphite liquor containing, say,

1% combined S in the form of sodium bisulphite and free S0 as such liquor yields pulp of very low ash content, this characteristic being desired by some manufacturers of cellulose derivatives. In certain instances, a cooking liquor containing a higher percentage of combined S0 than that usually maintained during sulphite digestion, say, one .containingf. 2% to 4% combined S0 and 4% to 5% free S0 or one containing equal pro rtion of 4% or 5% combined and free S is preferably a fiber liberation, as such used for efiecting high-combined llquor produces strong, hard pulp desirable for papermaking purposes. The resulting fiber is washed and screened, as usual, prior to treatment by our process.

The washed pulp is then treated at about 4% to 5% stock densityat about room temperature with chlorine water containing about 2% to 4% (all chemical percentages herein given are based upon weight of dry fiber) chlorine, depending upon the degree to which the raw stock has been cooked g, The chlorine reacts rapidly upon substantially all the ligneous content of the pulp,--that is, to the extent necessary for the realization of optimum results, but without materially degrading its alpha cellulose content. In lieu of treating the pulp with chlorine water, we may treat it at a stock density of about at about room temperature or lower, with bleach liquor of sulficient strength to pass the critical point only slightly. When so treating, however, the bleach liquor must be maintained alkaline during the entire treatment, as this apparently'represses the oxidizing reactio n,that is, renders it more selective upon the ligneous matter,"which is more susceptible treatment may reagents and under various conditlons. For

'Treatment in out in about five to eight hours at about atto oxidation than the c ellulose,so that treatment slightly beyond the critical point becomes more easily controllable. This may be accomplished by the addition of a suitable proportion of an alkali, say, about 1% to2% caustic soda, to the bleach liquor. The effect desired may also be realized by reacting upon a portion of the ligneous content of the pulp.

by chlorination, and another portion by oxidation, during which oxidation treatment be maintained to an alkaline condition ma promote a greater selectivity of reaction upon the ligneous matter, as hereinbefore described. For instance, undercooked may first be treated at ordinary temperatures at a 4% density with about 2 chlorine in water, until practically all the chlorine is consumed, then washed, and treated at a 10% stock densitywith a liquor containing 4% sodium or calcium base bleach, which ma contain to 1% caustic soda,

based on fiber, until practically all the bleach is consumed. Or the order of the chlorine and bleach treatments may be reversed,or the chlorinating and oxidizing treatments may be simultaneously carried out, erable to separate the steps, because of the easier control and more gentle treatment thereby realized.

After the chlorinating and/or oxidizing treatment, substantially all the ligneous content of the pulp has been reacted upon to'produce products soluble in the alkaline 'di estion which is to follow. The pulp is was ed and then treated with an alkaline solution under conditions to effect a dissolution and removal of its non-alpha cellulose component, including less-resistant ,celluloses and the reaction products of the chlorinating and/or oxidizing treatment. The alkaline be carried out with various instance, it may be carried out at a stock density of about 10% in a liquor containing about 7% caustic soda at atmospheric-boiling tem-, perature. Such treatment is complete. in about six hours, yielding a pulp of high alpha cellulose content. The treatment may also be carried out at a stock density of about 10% in a liquor containing 100% caustic soda the raw pulp when.

but it is prefion or its equivalent in the form-of sodium sul-,

phide, at about room temperature. Such latter treatment is accompanied by a higher rate of reaction, the treatment being complete in about two hours. Again, the treating liquor may be one containing about 10% lime, lime liquor should be carried mospheric-boiling temperature.

Most of the ligneous and other colormg .cortent of the pulp isremoved by the initial serves to whiten it substantially to ultimate whiteness, that is, to the point desired for and, after dilution to the proper consistency,

paper manufacture or for conversion into cellulose derivatives. Bleaching may be effected in a liquor containing 1% sodium or may be run oil on a paper machine into drier sheet, so called, in which form the pulp may be marketed for use as a raw material in the manufacture of paper, rayon, or other cellulose derivatives or products.

In some cases, particularly in preparing high alpha cellulose fiber of exceptional freedom from coloring matter and hence preferable for conversion into esters and other cellulose products of high quality, it is expedient purposely to use an excess of bleach in the final treatment, say, 4% or 5%, even though a high whiteness can be obtained with but 1% or 2%, for the use of excess bleach ensures the removal of traces of coloring matter evident only after the pulp has been esterified. In order ,to prevent contamination of the bleached product with oxycelluloses, ormore of free caustic soda should be present during the final bleaching step.

When free alkali is present during the final. step, it is advantageous in some-cases to use a high temperature in order further to reduce the beta and gamma celluloses and residual coloring matter. Excellent results may be obtained by raising the temperature to above, say, 150 F., preferably after most of the bleach has been exhausted. Insome cases it is expedient to hasten the further removal of beta and gamma celluloses and of residual coloring matter in the last .step by using, say, 1% to 4% bleach in the presence of a large excess of caustic.

We present herewith a table showing the average value of certain characteristics of a pulp prior to and after treatment by our process:

Bleached alphth fiber hour) 150 290-300 Pale cream. 105 if? 93%;;- 0 0 00 per number 3 11.3 S a solubility (3% boiling NaOH solution for 1 16% 3% to 5% The paramount advantage of the process is that we are enabled to produce a product bleached substantially to ultimate whiteness and of minimum oxycellulose content, as attested by its very low copper numiber, its low soda solubility, higher ouprammonium viscosity for a given degree of whiteness, and high color stability. Such a product is eminently satisfactory for conversion into high grade papers of marked permanency and into cellulose derivatives such as nitrocelluloses, cellulose xanthate, and other esters.

While the reagents and conditions of the treatments herein given are subject to variation, the initial treatment in any case should be carried out in a manner to effect a selective reaction u on substantially all the ligneous content 0 the pulp, but without materially degrading its alpha cellulose content, so that final bleaching may be accomplished under conditions to produce a white alpha pulp of very low oxycellulose content. The pretreated pulp will still contain material quantities of coloring matter prior to the alkaline digestion, but these will be largely removed by the alkali, so that a very white pulp can be finally realized through the use of small quantities of bleaching agent. In the various treatments, sodium or calcium base chemicals, either alone or in admixture, may be used, and the density of the pulp and the temperature of the various treatments may be controlled to produce the de= sired, product. The bleached pulp may, in certain instances, be advantageously selected or classified into a long fiber portion and a short fiber portion,the latter, portion containing the short and broken fiber and the fabrillae, which usually entrain therewith colloidal and coloring substance. We have found that the longer fiber portion is of particular advantage, not only for the manufacture of fine grade writing apers, but also for the preparation of cellu ose derivatives pared from a good grade of cotton linte-rs h;

The shorter fiber may be employed in a furnish to be used for the manufacture of relatively cheap paper. In certain instances, this selection or classification may be performed on the raw stock, and the long-fibered portion processed as hereinbefore described. After processing, the long-fi'bered productmay, if desired, be again subjected to classification, to ensure the separation of its less desirable portion. Owing to the lack of a better generic term,

by the use of the term lignin-reactiv'e re-. f

agent in the appended claims, we meani',

chlorine Water and/or bleach or equivalent reagents capable of reacting with the ligneous and other coloring material in raw pulp.

We claim:

1. A process which comprises separating from raw pulp its long fiber portion, treating the separated portion of pulp with a ligninreactive reagent and terminating the treatment slightly after a critical point is reached at which further consumption of the reagent is accompanied by a marked decrease in alpha cellulose and an increase in copper number, digesting the treated pulp in alkaline liquor, bleaching the alkaline-digested pulp, and classifying the bleached pulp into long fiber and short fiber portions.

2. A process which comprises separating from raw pulp its long fiber portion, treating the separated portion of pulp with a lignin-reactive reagent and terminating the treatment slightlyafter a critical point is reached at which further consumption of the reagent is accompanied by a marked decrease in alpha cellulose and an increase in copper number, digesting the treated pulp in alkaline liquor, and bleaching the alkaline-digested pulp.

3. A process which comprises treating raw pulp with. a lignin-reactive reagent and ter- 1. iinating the treatment slightly after a critical point is reached at which further consumption of the reagent is accompanied by a marked decrease in alpha cellulose and an increase in copper number, digesting the treated pulp in alkaline liquor, bleaching the alkaline-digested 'pulp, and classifying the bleached pulp into long fiber and short fiber portions.

In testimony whereof we have aflixed our signatures.

GEORGE A. RICHTER. MILTON O. SCHUR. 

